Add C++ sample code for client auth

[forderud]

There's some work-in-progress code on the https://github.com/forderud/WindowsClientAuth/tree/cpp-sample branch.

Doc: https://docs.microsoft.com/en-us/windows/win32/seccrypto/managing-certificates-with-certificate-stores

Windows 10 SDK search (C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0):

• X509Store: Zero hits
• X509Cert: Almost only hits in um\certenroll.h

[forderud]

Completed in 1ba1915a22662cda01043d5f76a72d45c45b75df.